1. Field of the Invention
The present invention relates to feed means for a high frequency or radio frequency (abbreviated as RF) heating device which heats an object such as food by high frequency dielectric heating, and more particularly to the prevention of leakage of higher harmonic electromagnetic wave components other than a fundamental frequency electromagnetic wave component used for the heating purpose.
2. Description of the Related Art
A frequency band permitted for use in an R.F. heating device is only permitted to use a specific frequency band (usually called an ISM band), although the band may differ from country to country, such as the 915 MHz band, the 2450 MHz band, etc. So long as there is no danger to the human body and safety is assured, there is no legal regulation on the frequency band. However, an RF oscillator usually generates higher harmonic components. In a magnetron, which is a microwave oscillator, oscillating at a fundamental frequency (fo) of 2450 MHz, relatively high power components are generated at 4900 MHz, 7350 MHz, 9800 MHz, and 12250 MHz, which are integral order higher harmonic components of the fundametal frequency (those components are represented by 2fo, 3fo, 4fo and 5fo).
Those higher harmonic components are subject to severe legal regulation in order to prevent disturbance to communication equipment.
Accordingly, various approaches to suppress the higher harmonic components have been made. FIGS. 1 and 2 show schematic sectional views of prior art RF heating devices having such a kind of means. In FIG. 1, a wave guide 3 is used as means for coupling a rectangular heating chamber 1 formed by conductive walls to an RF oscillator 2. An object 4 to be heated is placed in the heating chamber on a plate 5 made of a low dielectric material. The heating chamber walls have exhaust holes 6, through which water vapor generated from the object 4 during the heating is exhausted, and air inlet holes 7, through which fresh air is supplied, and a door 8 through which the object 4 is taken in and out of the heating chamber 1.
RF electromagnetic energy including higher harmonic components generated by the RF oscillator 2, is directed to the heating chamber 1 through the wave guide 3.
Once the higher harmonic components are fed into the heating chamber 1, they are transmitted out of the RF heating device through many paths, such as the exhaust holes 6, air inlet holes 7 and clearances between the door 8 and the heating chamber walls. As a result, it is difficult to design electromagnetic wave leakage prevention means to be arranged around the door. Thus, in order to attenuate the higher harmonic components themselves of the RF wave fed into the heating chamber, conductive bars 9, 10 and 11 of different lengths are mounted in the wave guide 3 to form a resonator operating as a band-pass filter in order to prevent the transmission of higher harmonic components other than the fo component into the heating chamber (Japanese Examined Utility Model Publication No. 51-14514).
However, in the structure in which the conductive bars 9, 10 and 11 of different lengths project, the suppression frequency band is very narrow because the suppression frequency is determined by the projection length. In order to widen the suppression frequency band, the number of conductive bars may be increased. However, the conductive bars have to be spaced from each other by a predetermined distance in order to prevent electric discharge due to the concentration of RF wave energy. Accordingly, if the conductive bars are selected one for each higher harmonic component, the length of the wave guide increases and the overall construction of the device becomes complex and expensive.
In FIG. 2, conductive plates 12, 13 and 14 each thereof having a width along a center axis of the wave guide 3 are arranged at spatial intervals of approximately .lambda.g/2, where .lambda.g is the wavelength of the fo component in the wave guide, to form a three-dimensional resonator to prevent transmission of electromagnetic waves having frequencies other than fo. However, it is difficult to dispose such three-dimensional circuit elements, which resonate only at fo, in a closed wave guide. Further, since such elements are arranged on the axis of the wave guide where the electric field strength is highest, large RF currents flow through the conductive plates and leads to a loss of the fundamental frequency component.